These are quite common. A quantity of them turned up a few years ago, and they are readily available currently. It is a good way to get an example of the notes in good grade, as the issued £1 note is quite a scarce Type, and is rarely seen in VF or better.

The remainders turn up both with and without serial numbers, and there are also partially printed examples.

The £5 denomination is not known as an issued note, so the cancelled remainder is the only way to obtain an example of the note.
There is also a rare £5 proof dated 1920 recorded, pictured; and a £50 note. Other denominations may also turn up.

The £1 notes as issued notes are reasonably common in low grades, though still scarcer than the £1 remainder notes.

They are listed in the SCWPM, 16th edition.

I have seen them selling on eBay for between £45 and £100 (actual sale rather than just offer price), they tend to not sell if offered at a higher price. There appears to be no difference in value currently between the £1 and £5, though in time the £5 may fetch a higher price.